The overall aim of this grant is to understand the role of a novel extracellular matrix protein named nephronectin in kidney development. In previous work, we have shown that the vast majority of mice lacking the integrin a8 subunit fail to develop the metanephric kidney with the initial phenotype of a deficit in ingrowth of the ureteric bud into the metanephric mesenchyrne, the site of expression of this integrin. We have identified nephronectin as an extracellular matrix protein expressed in the ureteric bud that is associated with the integrin a8bl and is distributed appropriately to activate its ligand-dependent signaling pathways. Because the phenotype during metanephric development of mice lacking a8bl is very similar to those of mice lacking any of the constituents of the GDNF-GFRal-c-ret signaling pathway, where the ligand GDNF is expressed in the mesenchyme and the receptor subunits GFRal and c-ret are expressed in the ureteric bud, we will examine possible interactions between these two signaling pathways and test the hypothesis that absence of a8bl results in reduced efficiency of signaling through this pathway. We will identify the receptors that mediate interactions of cells with nephronectin and determine which of the cell-types in the developing kidney interact directly with this ECM protein. In previous work, ligand engagement of integrins by extracellular matrix constituents has been shown to regulate many aspects of development including cellular proliferation, migration, differentiation and survival through interactions with the cytoskeleton and by activation of cytoplasmic signaling pathways including the ERK and Jun kinases, PI-3 kinase, and the cdc-42/radc-rho family of G proteins. We will attempt to determine which of these pathways are activated by nephronectin-engagement of a8bl and which are compromised by the absence of a8bl in the metanephric mesenchyme.